In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric and water drains by gravity and suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The papermaking machine is, in essence, a water removal system. After being dried, the paper is run between drums that impart the desired smoothness. This process is referred to as calendering and the more times paper is calendered the smoother the finish of the paper becomes. To create glossy paper, uncoated paper may be coated with a paint-like product and buffed by rollers under very high pressure, to create a shiny appearance. This process is referred to as supercalendering. Additional varnish layers may be applied to paper during the printing process to provide a gloss surface on the paper.
Supercalendering processes can either be an on-machine continuous process or an off-machine batch process. It is used to improve the paper sheet's surface properties, such as smoothness and gloss (shininess), which are critical for high-quality printing paper. Like other papermaking machine cross direction processes, the supercalendering process is a two-dimensional (spatial and temporal) process. The process starts with unwinding the paper sheet from the reel at an unwinder. The paper sheet is then fed between a series of rolls that are arranged in a vertical stack. The rolls are typically arranged to alternate hard and soft with two consecutive soft rolls in the middle of the stack. The paper sheet passes out from the bottom of the stack and wound up on a reel.
In paper production various grades of paper having different surface gloss are produced to suit various applications. During paper production, it is desirable to periodically or continuously measure the gloss of the surface of the paper to ensure that the paper surface has the desired gloss. This is typically done immediately after supercalendering with a gloss sensor that can be scanned back and forth along the cross direction of the moving sheet.
Two gloss sensor standards have been developed in the paper industry. The first standard, outlined under DIN 54502, for regular gloss measurements specifies that the measurements are to be taken using an angle of 75° for the incident light beam from a line perpendicular to the measured surface. For high-gloss measurements, measurements are taken using an angle of 45° for the incident light beam from a line perpendicular to the surface to be measured. If measurements at both angles are to be made, two separate and distinct sensors are generally used. The second standard, outlined under TAPPI T480, specifies that the measurement is to be taken only using an angle of 75° for an incident light beam from a line perpendicular to the measured surface.
Conventional devices, for measuring the gloss of paper surfaces, utilize an optical system that measures the intensity of a beam of light reflected from the paper surface. Gloss sensors are described in U.S. Pat. No. 6,404,502 to Preston et al. and U.S. Pat. No. 6,507,403 to Belotserkovsky. Typically, the gloss of the paper surface is determined by comparing its reflectance to the reflectance of a known gloss standard, such as a glass tile having a polished surface with a known gloss. Alternatively, the average intensity of the pixels can be employed. Additional techniques for measuring gloss numbers are described in US Patent Application Numbers 2007/0103674 and 2007/0103688 both to Kuusela.
As illustrated in FIG. 3, in measuring the reflectance of the paper surface 10, light 12 of known intensity is projected onto the surface, and a sensor which is responsive to the intensity of light is positioned to measure the intensity of the reflected light 20 from the paper surface. The sensor includes a condensing optics 14 and a single photometric detector 16 that yields signals 18 that represent a single intensity value 18. The gloss level is calculated as the ratio of the reflecting light beam intensity to the intensity of the illuminating light beam. As is apparent, this method yields only one average gloss value for the illuminated area of the paper. Most products such as paper, board, painted surfaces, etc. exhibit micro-level gloss variations within the illuminated area which cannot be measured by prior art techniques. Because of this “internal” gloss variation, products with the same conventional gloss value can manifest different visual outlooks to a consumer. The art is in need of a gloss sensor that is capable of distinguishing micro-level differences in the gloss on the surfaces of paper and other products.